To investigate the role of SV40 T antigen in the initiation of cellular DNA synthesis, experiments will be carried out using chick erythrocyte nuclei in heterokaryons with mouse cells transformed by SV40 tsA mutants at permissive and nonpermissive temperatures. Experiments will also be carried out on thymidine kinase (TK)-deficient mouse and human cells transformed to the TK-positive phenotype by UV-irradiated herpes simplex virus (HSV). Using HSV ts mutants, attempts will be made: (1) to identify the early HSV genes required to turn on the resident HSV TK in the transformed cells, and (2) to learn whether the transformed cells contain HSV genes other than those coding for TK. Karyological studies in somatic cell hybrids will also be carried out to learn whether the HSV TK gene is integrated into a specific human chromosome.